Radon occurs naturally in the lower atmosphere as a consequence of the continuous radioactive decay of its longer-lived precursors in the soil and the eventual diffusion of the radon into the atmosphere. Radon is emitted from its precursors as a gas and can consist of one of three isotopes: .sup.222 Rn which occurs during the uranium disintegration series, .sup.220 Rn which occurs during the thorium disintegration series, and, lastly, .sup.219 Rn which occurs as a consequence of the actinium disintegration series. Of these three isotopes, .sup.222 Rn has the longest half-life at 3.823 days, and .sup.219 Rn has the shortest half-life at 3.92 seconds with .sup.220 Rn having an intermediate 51.5 second half-life. The transformation from the radon isotopes to the next lower atomic weight substance in the respective disintegration series is accompanied by the emission of energetic alpha particles, and, to a lesser extent, by beta particles.
It has been recognized that radon can accumulate in enclosed spaces and that prolonged exposure to radon can pose a health hazard. Various devices have been developed for the purpose of measuring radon concentrations in residential and commercial buildings with one goal being the time integration of low-level concentrations in the ambient air. Such devices include charcoal collectors, alpha-track detectors, and electret detectors. Alpha-track detectors employ an emulsion deposited on a film-like substrate. The emulsion is sensitive to the through passage of alpha particles. After the emulsion is developed or otherwise processed, small tracks created in the emulsion can be counted as an indication of the radon concentration. Electret detectors utilize a charged electret to attract ions formed by the passage of alpha particles in an enclosed space. The reduction in charge consequent to ion formation over time is an accurate indication of radon concentration. Representative electret detectors are disclosed in U.S. Pat. No. 4,992,658 entitled "An Electret Ion Chamber for Radon Monitoring" and U.S. Pat. No. 4,853,536 entitled "Ionization Chamber for Monitoring Radioactive Gas." These detectors are of the integrating type in that they continuously measure the average concentrations of radon gas over a period of several days or weeks.
Since electret detectors are responsive to ion concentration, they are also sensitive to gamma radiation. In most cases, the number of ions generated by background gamma radiation is small compared to those generated by radon decay. However, in certain parts of the world, the background gamma radiation can be high enough to affect the accuracy of the radon measurement. For example, uranium mining tailing, which produces both radon and higher than average gamma radiation, has been used as a construction material in the Grand Junction, Colo. area of the United States. Likewise, naturally occurring deposits of an organic shale, known as Swedish Kolm, have a high, natural gamma radiation characteristic. This material is found in the Scandinavian countries and has been used as a construction material. Accordingly, the accuracy of radon measurements by electret detectors in buildings constructed of these materials or in geographical areas having high natural gamma radiation levels can be adversely affected because of the higher than average gamma radiation contribution to the measurement.
As can be appreciated, a need exists for a radon measurement system that compensates for or otherwise takes into account the gamma radiation contribution to the radon measurement.